1) Field of the Invention
The present invention relates to a wavelength superimposing device, a manufacturing method therefor, and a wavelength division multiplexing (WDM) network system, and, for example, relates to a technique suitable for use in a system in which a wavelength division multiplexing-passive optical network (WDM-PON) is adopted.
2) Description of the Related Art
Nowadays, as a subscriber optical fiber network system intended for subscriber (user) homes such as ordinary households, for example, a system connecting an optical line terminal (station) provided in a regional office set up in a central station or the like and optical network units set up in a plurality of subscriber homes using optical fibers is known. Among others, a configuration in which one optical fiber that performs input/output of an optical data signal (frame) from a regional office is branched off in a plurality of destinations by a power splitter, which is a passive element, and an optical network unit of each subscriber home is connected to each of the branched optical fibers is called a passive optical network (PON) system.
The PON system has been in practical use as a system that can perform data transmission at high speed between an optical line terminal and a plurality of subscribers' houses.
Then, a system configuration shown, for example, in FIG. 15 can be cited as a form of the PON system.
A PON system 300 shown in FIG. 15 is comprised of an optical line terminal (OLT) 100, N (N is an integer equal to or greater than 2) optical network units (ONU) 200-1 to 200-N (denoted simply as an ONU 200 if not distinguished) corresponding to subscribers #1 to #N, a power splitter 102, an optical fiber 400 connecting the OLT 100 and the power splitter 102, optical fibers 500-1 to 500-N (denoted simply as an optical fiber 500 if not distinguished) connecting the power splitter 102 and each of the ONU 200-1 to 200-N, and a wavelength filter 101 provided at some midpoint of the optical fiber 400.
In the PON system 300, the OLT 100 is a device equipped with required communication control functions such as converting an electric signal into an optical signal to send the optical signal in a predetermined downstream frame format (hereinafter referred to simply as a “downstream frame”) to the ONU 200 for delivery of information or the like or converting data transmitted as an optical signal in a predetermined upstream frame format (hereinafter referred to simply as a “upstream frame”) from the ONU 200 into an electric signal. Meanwhile, a downstream direction is from the OLT 100 to the ONU 200 and an upstream direction is from the ONU 200 to the OLT 100.
The optical fiber 400 connected to the OLT 100 is branched off by the power splitter 102 provided midway through a transmission path and each branched optical fiber 500 is drawn into an individual subscriber home to be connected to each ONU 200.
The ONU 200 is a device that performs communication control operations such as communication with the OLT 100 and conversion between optical signals and electric signals.
The power splitter 102 divides a downstream signal from one optical fiber 400 into a plurality of optical fibers 500 (power branching) and collects (multiplexing) upstream signals from a plurality of optical fibers 500 into one optical fiber 400.
Here, upstream and downstream data transmission between the OLT 100 and the power splitter 102 is performed bidirectionally by wavelength division multiplexing (WDM) using one optical fiber 400.
The wavelength filter 101 is used to transmit data in downstream frames and upstream frames while multiplexing an analog video signal transmitted (broadcasted) uniformly to the subscribers #1 to #N into an optical signal in the downstream direction.
In the downstream direction from the OLT 100 to the ONU 200, for example, video signal light in a 1.55 μm band to be multiplexed by the wavelength filter 101 is transmitted along with an optical signal in a 1.49 μm band by time division multiplexing (TDM). Then, in the ONU 200, after demultiplexing a downstream frame and the video signal light by, for example, a wavelength filter (not shown), the video signal light is received by, for example, a receiver for video signal light (not shown) and frame synchronization information and management information are detected in the downstream frame, and based on the detected information, reception processing is performed by extracting data of time slots individually assigned in advance.
In the upstream direction from the ONU 200 to the OLT 100, on the other hand, an upstream frame from each ONU 200 is transmitted as an optical signal in a 1.31 μm band by time division multiple access (TDMA) at a timing to avoid collision of each frame. Each ONU 200 is notified of access timing of TDMA by, for example, the management information.
As optical devices that are applicable instead of the power splitter 102 in the PON system 300, techniques proposed in Japanese Patent Application Laid-Open No. 2005-321487 and Japanese Patent Application Laid-Open No. 2001-021741 shown below exist.
Now, with the increasing number of subscribers in recent years, further improvement in speed and broader bands of the PON system 300 have been demanded. Thus, as a next-generation PON system, a WDM-PON system using WDM that performs user (subscriber) multiplexing on an wavelength axis in the downstream direction and upstream direction respectively is considered.
Since, in the WDM-PON system, a different wavelength is assigned to each user, instead of the power splitter 102 used in the PON system 300 described above, for example, a multiplexing/demultiplexing device 600 shown in FIG. 16(a) or a multiplexing/demultiplexing device 601 shown in FIG. 16(b) is used. Though an illustration of the OLT 100 and ONU 200 is not shown in FIG. 16(a) and FIG. 16(b), the OLT 100 is for example connected to both the multiplexing/demultiplexing devices 600 and 601 to the left of the page and each ONU 200 is connected to the right of the page.
The multiplexing/demultiplexing device 600 shown in FIG. 16(a) is comprised of, for example, a coarse wavelength division multiplexing (CWDM) multi/demultiplexer 103 such as an arrayed waveguide grating (AWG) and a plurality of power splitters 104.
In the multiplexing/demultiplexing device 600, the CWDM multi/demultiplexer 103 is used to demultiplex an input light in which an optical signal of a plurality of wavelengths (for example, wavelengths λ1 to λ4 in the figure) is wavelength-multiplexed into each wavelength and, at the same time, to multiplex each optical signal of a plurality of wavelengths in an opposite direction of the input light to output a wavelength-multiplexed optical signal. The power splitters 104 are used to divide a downstream optical signal from the CWDM multi/demultiplexer 103 into ONU 200 (power branching) and, at the same time, to multiplex each upstream optical signal from the ONU 200.
By using the multiplexing/demultiplexing device 600 in the PON system 300 instead of the power splitter 102 as described above, in the example shown in FIG. 16(a), the number of wavelengths used for optical transmission can maximally be quadrupled to realize further improvement in speed and broader bands of the PON system 300 in response to the increasing number of users.
If, instead of the power splitter 102, the multiplexing/demultiplexing device 601 shown in FIG. 16(b) is used, a downstream optical signal from the OLT 100 in which an optical signal of a plurality of wavelengths (for example, wavelengths λ1 to λN in the figure) is wavelength-multiplexed is demultiplexed into each wavelength by a dense wavelength division multiplexing (DWDM) multi/demultiplexer 105 before being transmitted to each ONU 200 assigned in advance. Each upstream optical signal of a plurality of wavelengths from each ONU 200, on the other hand, is multiplexed by the DWDM multi/demultiplexer 105 before being transmitted to the OLT 100 as a wavelength-multiplexed optical signal.
If, instead of the power splitter 102, the multiplexing/demultiplexing device 601 is used in the PON system 300, as described above, since one wavelength can be assigned to each user, it becomes possible not only to achieve further improvement in speed and broader bands of the PON system 300 in response to the increasing number of users, but also to constitute a more flexible network system.
In addition, an optical waveguide WDM multi/demultiplexer proposed in Japanese Patent Application Laid-Open No. 2005-321487 is available.
However, if the power splitter 102 is replaced (upgraded) by the multiplexing/demultiplexing device 600 or 601 in the PON system 300, as described above, simple power branching of a video signal light cannot be performed due to the presence of the CWDM multi/demultiplexer 103 or DWDM multi/demultiplexer 105 having wavelength dependence so that broadcasting to each ONU 200 cannot be carried out as before. Thus, a method of making the PON system 300 faster and bands broader is needed while retaining a function of broadcasting a video signal light.
So, instead of the power splitter 102, for example, a multiplexing/demultiplexing device 602 shown in FIG. 17 can be used.
The multiplexing/demultiplexing device 602 is comprised of, for example, a WDM filter 106 which multiplexes/demultiplexes a wavelength-multiplexed light of wavelengths λ1 to λN and a video signal light of the wavelength λvideo, a WDM multi/demultiplexer 109 which multiplexes/demultiplexes a wavelength-multiplexed light in which light of the wavelengths λ1 to λN is wavelength-multiplexed, a power splitter 107 which divides a video signal light of the wavelength λvideo (power branching), and WDM multi/demultiplexers 108-1 to 108-N (denoted simply as a WDM multi/demultiplexer 108 when not distinguished) which multiplexes/demultiplexes a video signal light of the wavelength λvideo and one of light signals of the wavelengths λ1 to λN. Though an illustration of the OLT 100 and ONU 200 is not shown in the configuration shown in FIG. 17, for example, the OLT 100 is connected to the left of the page and each ONU 200 is connected to the right of the page.
With such a configuration, a wavelength-multiplexed light of the wavelengths λ1 to λN is demultiplexed into each wavelength by the WDM multi/demultiplexer 109 and a light of each wavelength is transmitted to the ONU 200 assigned in advance. A video signal light of the wavelength λvideo, on the other hand, is demultiplexed in the downstream direction by the WDM multi/demultiplexer 106 and further power-branched evenly by the power splitter 107, and then multiplexed with one of downstream optical signals of the wavelengths λ1 to λN by the WDM multi/demultiplexer 108 before being transmitted (broadcasted) to each ONU 200.
The video signal light of the wavelength λvideo is demultiplexed in the upstream direction by the WDM multi/demultiplexer 108 and further collected (multiplexed) by the power splitter 107, and then multiplexed with a wavelength-multiplexed optical signal in the upstream direction of the wavelengths λ1 to λN by the WDM multi/demultiplexer 106 before being transmitted to the OLT 100.
In addition, for example, an optical waveguide proposed in Japanese Patent Application Laid-Open No. 2001-021741 is available as a technique that can be applied to realize broadcasting of a video signal light in the PON system.
However, if the configuration shown in FIG. 17 is used, as described above, there is a splice problem in which discrete components (WDM filters 106 and 108) cannot be freely detached/attached and there is also a problem of increasing device size of the multiplexing/demultiplexing device 602 due to fiber remaining length handling, for example.
Also, if the technique described in Japanese Patent Application Laid-Open No. 2001-021741 is used, there is a problem that a transmission light loss increases (for example, the loss amounts to 10 dB or more) because an optical path length in an optical waveguide becomes longer.